The present invention relates to an injection molding machine using a linear motor.
An injection molding machine for molding a product by injecting a molten resin, etc. into a die assembly involves the use of a multiplicity of linear drive mechanisms as for an injection apparatus in which a screw and a plunger are moved in axial direction within a barrel, a clamping apparatus for opening and closing and clamping the die assembly.
This linear drive has hitherto usually employed a mechanism for converting a rotary motion of a motor into a linear motion through a ball screw. The mechanism for converting the rotary motion into the linear motion through the ball screw is, however, complicated, and, a contrivance for omitting this complicated mechanism, as disclosed in Japanese Patent Application Post-Exam Publication No. 5-13805, is that the linear motion unit is driven directly by use of an AC linear motor.
The use of the AC linear motor, however, entails problems arisen as follows:
(a) An axial length of the linear motor is longer than the ball screw, and hence the apparatus increases in size, with the result that an area for installing the apparatus is enlarged and this is followed by a rise in cost.
(b) The linear motor using an iron-cored coil is utilized for obtaining a large thrust, however, a magnetic attraction force that is several times as large as the thrust is generated, so that a highly rigid mechanical structure is needed with an increased weight of this structural body.
(c) An electrical angle origin lead-in operation is difficult due to the strong magnetic attraction force, and a phase error in electrical angle can not be reduced. This electrical angle phase error eventually becomes a factor of disturbance, and therefore a gain can not be raised, resulting in an occurrence of vibrations.
Such being the case, an injection molding machine using a DC linear motor having none these problems came to be employed.
In this injection molding machine using the DC linear motor, however, a new problem arises, wherein an exothermic quantity of the coil becomes large depending on a state of the use.
This heat involved seriously affects characteristics of a measuring device such as a position measuring linear scale installed in the vicinity of the motor, and besides, if the heat emission is intensive, this might cause serious troubles such as a burn down of the coil, and a subsequent fire and so on.
The heat emission might cause the serious troubles described above, and nevertheless there has hitherto existed a device for displaying an exothermic state or giving an alarm against the heat emission before the trouble happens.